I used to run 2" on the S16g to a stock 1st gen SMIC, then 2" out from the IC, then bumped up to 2.5" at the HRC stage1 piping about 18" before the TB. Worked fine. I then got the HRC universal FMIC, redid all my piping in the process. I used 2.25" from the turbo outlet to the IC inlet, 2.25" from the IC outlet to the same 2.5" HRC piping as before. I noticed a BIG gain in spoolup rpm(about 200 or so rpm sooner), and a gain in top end rpm power. Granted, this was only a butt dyno, but there was a definate noticable difference. Too large of piping before the IC, will *usually* hurt velocity up to the IC, and hurt your spoolup time. Too small diameter, and it hurts top end flow. If anything, keep the piping from the compressor outlet, to the IC inlet, smaller(turbine outlet size or .25" larger), and then up the piping size maybe .25" from the IC outlet to the TB inlet. It's best IMO, to up the size a little bit out of the IC, as the air is being cooled, it expands, much like exhaust gases in an exhaust system(granted they are much cooler temps). With the expanding air from the IC, and slightly larger piping size, velocity *should* remain constant in a perfect world. This is how i've seen many, many turbo setups run, ones that are not stock, and make good power anyway(all other things being equal). The best way to figure the best size to use for a particular setup, is to use a dyno.

13.5 @108 MPH-2.2 60ft(stupid FWD!)
S16G @ 18 PSI/FMIC/Running on MegaSquirt II (Now with sequential
fuel injection)
My webpage: http://eclipsed4evr.home.comcast.net
-1998 Mitsubishi Eclipse RS-T- "Toy"
-1992 Plymouth Laser Turbo AWD(SOLD)
-2000 Honda CR-V(daily)